Remote surveillance is accomplished in a cost effective manner by employing a high resolution charge coupled device (CCD) image sensor. The high resolution, cost effectiveness, and overall performance of these devices are based on imaging sensing technology that exploits the exceptional processing capabilities of current microcircuit lithographic techniques applied to compositions such as germanium, indium gallium arsenide, gallium antimonide, or, most frequently, crystalline silicon. These CCD image sensors find wide ranging applications in video camcorders, closed circuit televisions, and military applications. Image sensor components are manufactured in large quantities, and some of the consumer market applications like camcorders provide consumers with high performance imaging recorders at nominal costs.
To effectively perform surveillance and camcorder recording applications, the scenes of interest must be adequately illuminated. While natural daylight and artificial illumination are the obvious light choices, these illumination sources may not be the best choices for the type of imaging sensor being used. For example, silicon imaging sensors have a peak optical response at an approximate wavelength of 0.75 .mu.m to 1 .mu.m (infrared). However, the sun's maximum optical output occurs at a wavelength of about 0.5 .mu.m (visible) and efficient artificial light sources, which are designed to maximize light output in the spectral region where human eye response is greatest, have optical outputs at or about 0.5 .mu.m wavelength. As such, neither artificial nor natural light have optical outputs at or about the peak optical response of silicon imaging sensors.
Conventional sources of illumination, such as incandescent lamps in which light is emitted from a highly heated resistance wire and incandescent mantles of the Welsbach type, generally have characteristics of the "black body", or more realistically "gray body", type and emit radiation over a broad spectral band. Low pressure alkali metal vapor gas discharge lamps emit relatively narrow bands of radiation in the ultraviolet, visible, and near infrared range, depending on the alkali metal. The most significant commercial example of an alkali metal illumination system is the low pressure sodium lamp which has the highest efficacy (approaching 200 lumens/watt) of all available electrically-powered lamps. Low pressure sodium lamps emit intense radiation in the visible spectrum. However, when dealing with covert illumination, it is desirable that the light source emit almost entirely infrared radiation so that radiation visible to the human eye is not emitted and detection by third parties and individuals under surveillance is thereby avoided.
Selective emitters, i.e. superemitters, have been developed to produce radiation in relatively concentrated, narrow spectral bands for particular applications, such as the generation of electricity by thermophotovoltaic devices. As disclosed in U.S. Pat. No. 5,356,487, materials comprising the superemitters often have an element present in a mixed oxidation or mixed valence state, forming a nonstoichiometric oxide. Such materials include rare earth/alkaline earth oxide systems, rare earth/transition metal oxide system, and various other mixed metal oxide systems. U.S. Pat. No. 4,584,426 discloses the use of certain rare earth oxide radiators to provide radiant energy for thermophotovoltaic devices.
Thus, illumination sources conventionally used for surveillance applications compromise an imaging device's performance and/or application for covert surveillance because they emit radiation visible to the human eye or because they generate optical output far from the wavelength where the device is most responsive. For example, an optimal surveillance illuminator for a silicon CCD image sensor would be a wavelength-selective source that concentrates its optical output in a narrow band at a wavelength of about 1 .mu.m. The wavelength choice of about 1 .mu.m coincides with the maximum responsivity of the silicon CCD. Additionally, the emission of a narrow band of wavelengths enhances the efficiency of the light source because energy is not being wasted on generating optical output at wavelengths where the silicon light detector may not convert light to electricity with an acceptable conversion efficiency, or where the silicon light detector may not convert light to electricity at all, for example with wavelengths longer than 1.15 .mu.m.